In what follows, the basic function of known solar tower power plants or solar tower systems will be explained by means of FIG. 1 to FIG. 6.
FIG. 1 shows a solar tower power plant or solar tower system known from U.S. Pat. No. 4,172,443, having a tower 120, on which a receiver 110 is installed, onto which heliostats 190 concentrate solar radiation. The heliostat field 130 consists of a plurality of such heliostats 190. In other known towers, several receivers can even be installed on the same tower, as shown in EP 2000669 A2. As explained in [1]—especially on pages 27 et seq.—the concentrated radiation through the receiver heats a heat transfer agent, by which a turbine is driven, which then generates electric current via a mechanically coupled generator.
Present-day solar tower systems can be classified and characterized in four solar tower systems, as follows:
1. Solar tower system with surrounding heliostat field (far field), see schematic of a plan view in FIG. 2.
2. Solar tower system with polar field, see schematic of a plan view in FIG. 3.
3. Solar tower system with heliostat field underneath the receiver (near field), see schematic of a plan view in FIG. 4 and in perspective view in FIG. 5.
4. Solar tower system with north and south field, see schematic of a plan view in FIG. 6.
The systems mentioned in FIG. 2 to FIG. 6 shall be described in greater detail below.
1. Solar Tower Systems with Surrounding Heliostat Field (Far Field)
Most (commercial) solar tower systems consist of a cylindrical or inverted frustum-shaped receiver oriented 360° in all directions of the sky, being supported by a central tower and surrounded by a heliostat field 130 formed from individual heliostats 190, as shown in FIG. 1 and as plan view in FIG. 2. A cylindrical receiver or an inverted frustum receiver are receivers with external absorber, in which the envelope surface of the receiver forms the absorber surface. In FIG. 1, a solar tower system known from U.S. Pat. No. 4,172,443 is shown, where a cylindrical receiver 110, whose outer cylindrical envelope forms the absorber surface, is situated on a tower 120 at a receiver height HR above the heliostat field 130.
FIG. 2 shows the schematic of a plan view of a known solar tower system with a heliostat field 230 surrounding the tower 210 at a distance. The heliostat field 230 has the shape of a ring, where the region 234 around the tower 220 that supports the receiver 210 is free, i.e., no heliostats are arranged in the region 234. The position of the tower with the receiver often does not lie in the exact center, but instead is displaced in the direction of the equator with respect to the heliostat field, i.e., to the south in the northern hemisphere or to the north in the southern hemisphere.
In FIG. 2, the heliostat field consists of a far field. The far field is a heliostat field which—as distinguished from the near field defined further below—surrounds the tower and the receiver with a certain horizontal distance, and whose reflector surface density ρ decreases with increasing distance from the receiver. The reflector surface density ρ is defined as the ratio of the reflector surface of the heliostat field to the ground surface of the heliostat field. FIG. 2 shows the region 234 near the receiver 210 where no heliostats are installed.
Instead of being cylindrical or a continuous frustum, the receiver can also consist of a plurality of individual receivers.
The receiver height HR, as shown in FIG. 1, is defined as the vertical distance of the midpoint of the absorber surface of a receiver with external absorber or the receiver aperture of a hohlraum receiver from the plane that is defined through the midpoints of the reflectors of the heliostats of the heliostat field. The receiver height HR is used hereinafter as a unit quantity against which other quantities can be measured, such as the heliostat field size.
The diameter DH of a heliostat field, as shown in FIGS. 2, 3, 4 and 6, is defined as the distance of the heliostats furthest away from each other.
Solar tower systems with surrounding heliostat field typically have receiver heights HR of more than 100 m and heliostat fields with a diameter of more than eight receiver heights, i.e., DH>8×HR. For example, the Gemasolar solar tower described in [2] has a receiver height HR of 140 m and a diameter DH of around 1200 m. For example, the solar tower of Solar-Reserve, described in [3], has a receiver height HR=182.88 m (600 feet) and DH=2600 m.
2. Solar Tower Systems with Polar Field
As can be seen in the schematic of the plan view of a solar tower system with polar field in FIG. 3, this has a heliostat field 330 only on the polar side—pointing in the northerly direction in the northern hemisphere and in the southerly direction in the southern hemisphere—of the tower 320 and receiver 310 and, as shown EP 2000669 A2, with one or more receivers 310 on the tower 320, oriented to the heliostat field.
Like the far field of a solar tower with surrounding heliostat field, the reflector surface density ρ of the polar field increases with increasing distance from the receiver.
Solar tower systems with polar field typically have receiver heights of 50-150 m and heliostat fields with a diameter DH of around five to six receiver heights, where 3×HR<DH<7×HR.
For example, the “Solar Tower Jülich” described in [4] has a receiver height HR of 55 m and a diameter DH of the heliostat field of around 300 m.
For example, the “PS10 Solar Tower” described in [5] has a receiver height HR=115 m and a diameter DH=750 m, the “PS20 Solar Tower” has HR=165 m and DH=1000 m.
3. Solar Tower System with Heliostat Field Underneath the Receiver (Near Field)
In the 1960s, a first solar tower system was developed by Giovanni Francia in Italy where the heliostat field is located underneath the downwardly directed receiver and extends in the north, south, east and west direction. This is depicted in FIG. 4, where the heliostat field 430 and the position of the receiver 410 are seen in plan view. Unlike the above-discussed far field, the heliostat field is a near field in which the heliostats are mounted with constant reflector surface density ρ. The receiver 510 is mounted suspended from a jib system 520, as shown in FIG. 5. For further explanations, see page 238 in [1] and U.S. Pat. No. 4,220,140. From FIG. 7.77 on page 238 in [1] one can see that the heliostat field of round heliostats has a quite large reflector surface density ρ of around 60%, which cannot be achieved with rectangular heliostats—on this, see page 706 in [8]. No further detailed published data on the reflector surface density ρ of the solar tower system of Giovanni Francia is known.
The solar tower systems developed by Giovanni Francia with heliostat field underneath the receiver typically have receiver heights HR of less than 20 m and heliostat fields with a diameter DH of less than two receiver heights HR, i.e., DH<2×HR, as can be seen from FIG. 7.77 on page 238 in [1]. No further detailed published data is known.
4. Solar Tower System with North and South Field
A further solar tower system described in [7] which has been developed by the firm eSolar Inc., homepage and main office see [6], is a mixture of a solar tower system with surrounding heliostat field and a solar tower system with polar field. As can be seen from the plan view in FIG. 6, it consists of a tower 620, on which is situated a receiver 610 with two apertures, and a north field 631 and a south field 632, which together nearly surround the tower 620, like a surrounding heliostat field. In this case, the single receiver 610 has two apertures, one directed toward the north field and the other toward the south field, as described more closely in [7].
Characteristic of this heliostat field of the solar tower system from eSolar Inc. is the uniform reflector surface density ρ of the entire heliostat field. See WO 2008/154521 A1. However, the heliostat field of eSolar Inc. differs from the solar tower system with near field of Giovanni Francia in that the heliostat field is not located underneath the receiver and the reflector surface density ρ is less than 50%.
Solar tower systems with north and south field from eSolar Inc. typically have receiver heights HR of around 50 m and heliostat fields with a diameter DH of around five receiver heights, that is: DH=5×HR, as described further in [7].
Other known technologies for solar tower system are the most used heliostats with rigid vertical axle suspension (FVA) and the known but usually not used heliostats with rigid horizontal axle suspension (FHA), which will be explained by means of FIG. 7 to FIG. 10.
Heliostats with Rigid Horizontal Axle Suspension (FHA)
In WO 02/070966 A1, WO 2008/092194 A1, WO 2008/092195 A1 and [8], heliostats with rigid horizontal axle suspension (FHA) are described. Heliostats with FHA differ from conventional heliostats, which have a rigid vertical axle suspension (FVA), by the spatial volume in which the reflectors can move freely on account of their suspension.
A heliostat has a first rotary axle and a second rotary axle, arranged perpendicular to the first, and it is arranged on a mounting surface, where the first rotary axle is fixed relative to the mounting surface and the second rotary axle is fixed relative to the reflector.
In FIG. 7 are shown representations of the principle of a heliostat with FVA, known from [8]. In FIG. 7a is shown the principle of a heliostat with FVA and in FIG. 7b a sample rectangular heliostat, and in FIG. 7c the corresponding spatial volume in which the reflector of the heliostat can move freely on account of the suspension. As can be seen in FIG. 7a and FIG. 7b, in a heliostat with FVA the first rotary axle 792 rigidly connected to the mounting surface or ground surface is vertical or perpendicular to the mounting surface or ground surface, while the second rotary axle 793, perpendicular to the first, can move about the first rotary axle 792.
In FIG. 7c is shown the spatial volume 799 in which the reflector can move freely on account of the suspensions. The spatial volume is a semicircular body, corresponding to a layer of a sphere in which the surfaces are located at top and bottom, i.e., perpendicular to the first vertical rotary axle 792 rigidly joined to the mounting surface. In FIG. 13 in WO 2008/092195 A1 and the corresponding discussion, a heliostat with FVA and corresponding spatial volume in which the reflector of the heliostat can move freely on account of the suspension is depicted and explained.
FIG. 8 from [8] shows at what distances one can set up heliostats 890 with FVA without overlapping of the spatial volumes 899 in which the reflectors of the heliostat 890 can move freely, so as to avoid collision between the heliostats.
The known manufacturers of heliostats use heliostats with FVA.
In a heliostat with FHA, the first rotary axle 992 rigidly joined to the mounting surface is parallel to the mounting surface or ground surface, as can be seen in the system representation of FIG. 9a. The first rigid horizontal rotary axle 992 in the heliostat with FHA is rigidly joined to the ground, as shown in WO 2008/092194 A1 and WO 2008/092195 A1, while the second rotary axle 993, perpendicular to the first, can turn about the first rotary axle 992.
As can be seen in FIG. 9b, the spatial volume 999 in which the reflector of a heliostat with FHA can move freely is equal to that of the heliostat with FVA, but rotated by 90°, so that the rotary axle 992 rigidly joined parallel to the mounting surface is perpendicular to the surfaces of the semicircular body. In FIG. 12 of WO 2008/092195 A1 and the accompanying discussion, a heliostat with FHA and the corresponding spatial volume in which the reflector of the heliostat can move freely on account of the suspension are explained.
As can be seen in FIG. 10 and as known from [8], heliostats with FHA can be set up close together in rows without overlapping of the spatial volumes 1099 in which the reflectors of adjacent heliostats 1090 can move freely. In FIG. 10 the theoretically maximum reflector surface density for the given reflector size is shown, with no safety margin between adjacent heliostats. Furthermore, the rows are staggered so as to place them as close together as possible.
It should be recognized that heliostats with FHA enable higher reflector surface densities than heliostats with FVA. As shown in FIG. 10 in [8] and deduced from the accompanying discussions, the maximum possible reflector surface density of rectangular heliostats with FVA in the ideal case is around 58%, while rectangular heliostats with FHA enable substantially higher reflector surface densities, with theoretically up to almost 100%. As is known from [8], the theoretically possible maximum reflector surface density ρ of a heliostat field increases when each reflector 995 is longer in the direction of the second rotary axle 993 than in the direction perpendicular to this.
In FIG. 6 in WO 02/070966 A1 and the accompanying discussion, the mechanical coupling of heliostats with FHA is shown.
Furthermore, different receiver technologies are known, especially the Hohlraum receiver shown in FIG. 11.
Receivers
There are receivers, as shown in FIG. 1, where the envelope surface constitutes the absorber surface, as already discussed. Other receivers have a target surface, i.e., an aperture or absorber surface, whose surface normals are oriented essentially in the same direction.
In FIG. 11 one sees the cross section of a Hohlraum receiver known from [9]. Concentrated solar radiation enters the Hohlraum receiver through the aperture 1111 and impinges on the absorber 1115, where the heat is surrendered to a heat transfer agent. In the system shown, the heat transfer agent is air, which enters the receiver through the inlet 1117 and, being heated, leaves through the outlet 1118. This kind of receiver, furthermore, has a glass dome 1113 to keep the air in the receiver. This principle of a Hohlraum receiver is also known from the patent document U.S. Pat. No. 4,220,140 and WO 2008/153922 A1.